BUG: renable delete, fixing selection node and compare view

From feedback from Ron:
- the delete buttons were disabled by mistake, re-enabled them and
  added a right click menu option to delete highlighted markups
- don't offer the move to another list right click menu if there
  are no other lists
- fixed the selection node observations so that could make the
  fiducial list in the Neuro tutorial test active when it was
  run as a self test in Slicer
- added more error checking to avoid crashes in moving between
  compare view layouts
- swap out the seed handles between 2d and 3d ones as move between
  compare and non compare view layouts
- still some debugging to do in compare view